Driver circuitry for piezoelectric transducers

ABSTRACT

The present disclosure relates to circuitry for driving a piezoelectric transducer based on an input signal. The circuitry comprises: primary driver circuitry configured to receive the input signal and to output a primary driving signal to the piezoelectric transducer based on the input signal; and secondary driver circuitry configured to receive an error signal indicative of an error between the input signal and the primary driving signal and to output a secondary driving signal to the piezoelectric transducer based on the error signal, wherein the primary driver circuitry and the secondary driver circuitry both comprise switching converter circuitry.

FIELD OF THE INVENTION

The present disclosure relates to driver circuitry for piezoelectrictransducers.

BACKGROUND

Piezoelectric transducers are increasingly being seen as a viablealternative to transducers such as speakers and resonant actuators forproviding audio and/or haptic outputs in devices such as mobiletelephones, laptop and tablet computers and the like, due to their thinform factor, which may be beneficial in meeting the demand forincreasing functionality in such devices without significantlyincreasing their size. Piezoelectric transducers are also increasinglyfinding application as transducers for ultrasonic sensing andrange-finding systems.

Piezoelectric transducers can be voltage-driven. However, when driven byvoltage piezoelectric transducers exhibit both hysteresis and creep,which means that when driven by voltage the displacement of apiezoelectric transducer depends on both the currently-applied voltageand on a previously-applied voltage. Thus, for any given driving voltagethere are multiple possible displacements of the piezoelectrictransducer. For audio applications this manifests as distortion.

One way of reducing hysteresis and creep and the associated problems ina piezoelectric transducers is to drive the transducer with chargeinstead of voltage. When driven with charge, the displacement of thepiezoelectric transducer varies with the charge applied.

FIG. 1 is a schematic illustration of circuitry for driving apiezoelectric transducer with charge. As shown generally at 100 in FIG.1, charge drive circuitry 102, which may be charge pump circuitry, forexample, may receive an electrical input signal (e.g. an input audio orultrasonic signal or haptic waveform) from upstream circuitry (notshown) such as amplifier circuitry, and drive a piezoelectric transducer104 to cause the piezoelectric transducer 104 to produce an audible,ultrasonic or haptic output based on the electrical input signal.

SUMMARY

According to a first aspect, the invention provides circuitry fordriving a piezoelectric transducer based on an input signal, thecircuitry comprising:

-   -   primary driver circuitry configured to receive the input signal        and to output a primary driving signal to the piezoelectric        transducer based on the input signal; and    -   secondary driver circuitry configured to receive an error signal        indicative of an error between the input signal and the primary        driving signal and to output a secondary driving signal to the        piezoelectric transducer based on the error signal,    -   wherein the primary driver circuitry and the secondary driver        circuitry both comprise switching converter circuitry.

In one example, the primary driver circuitry comprises variable voltagepower supply circuitry and the secondary driver circuitry comprisescharge pump circuitry.

The variable voltage power supply circuitry may comprise a switchnetwork, an inductor and a reservoir capacitor.

The charge pump circuitry may comprise a switch network and a flyingcapacitor.

The flying capacitor may be variable.

The charge pump circuitry may be configured to receive a power supplythat varies based on a parameter of the input signal.

The variable voltage power supply circuitry may be configured to providethe power supply to the charge pump circuitry.

The variable voltage power supply circuitry may comprise detectorcircuitry configured to detect a level, envelope or other parameter ofthe input signal and to control the power supply voltage provided to thecharge pump circuitry based on the detected level, envelope or otherparameter.

The charge pump circuitry may comprise one or more supply capacitors,and the switch network may be operable to transfer charge between thereservoir capacitor and the one or more supply capacitors.

In another example, the primary driver circuitry comprises firstvariable voltage power supply circuitry and the secondary drivercircuitry comprises second variable voltage power supply circuitry.

The primary driver circuitry and the secondary driver circuitry may eachcomprise a switch network and an inductor, and the inductor of thesecondary driver circuitry may be smaller than the inductor of theprimary driver circuitry.

The inductor of the secondary driver circuitry may be embedded inintegrated circuitry that implements the circuitry.

The first variable voltage power supply circuitry may comprise a firstreservoir capacitor for storing charge.

The first reservoir capacitor may be shared by the first variablevoltage power supply circuitry and the second variable voltage powersupply circuitry.

The second variable voltage power supply circuitry may comprise a secondreservoir capacitor for storing charge.

The circuitry may further comprise a helper capacitor configured toreceive charge from the primary driver circuitry or the secondary drivercircuitry in order to adjust a voltage across the helper capacitor.

The Helper Capacitor may be Coupled:

-   -   in series between the piezoelectric transducer and ground; or.    -   in series between an output of the secondary driver circuitry        and a first terminal of the piezoelectric transducer; or    -   in parallel with the piezoelectric transducer, between an output        of the secondary driver circuitry and ground; or    -   in parallel with the piezoelectric transducer, between an output        of the primary driver circuitry and ground.

The Circuitry may Comprise:

-   -   a first helper capacitor coupled in parallel with the        piezoelectric transducer, between an output of the secondary        driver circuitry and ground; and    -   a second helper capacitor coupled in parallel with the        piezoelectric transducer, between an output of the primary        driver circuitry and ground.

The Circuitry may Comprise:

-   -   a first helper capacitor coupled in parallel with the        piezoelectric transducer, between an output of the secondary        driver circuitry and ground; and    -   a second helper capacitor coupled in in series between an output        of the secondary driver circuitry and a first terminal of the        piezoelectric transducer.

The primary driver circuitry may be configured to be coupled to aterminal of the piezoelectric transducer and the secondary drivercircuitry may be configured to be coupled to the same terminal of thepiezoelectric transducer.

Alternatively, the primary driver circuitry may be configured to becoupled to a first terminal of the piezoelectric transducer and thesecondary driver circuitry may be configured to be coupled to a secondterminal of the piezoelectric transducer.

The circuitry may further comprise commutation circuitry configured toselectively couple one of a first terminal and a second terminal of thepiezoelectric transducer to the primary driver circuitry and thesecondary driver circuitry, and to couple the other of the firstterminal and the second terminal of the piezoelectric transducer to areference voltage supply.

The commutation circuitry may be configured to selectively couple one ofa first terminal and a second terminal of the piezoelectric transducerto the primary driver circuitry and the secondary driver circuitry, andto couple the other of the first terminal and the second terminal of thepiezoelectric transducer to a reference voltage supply based on apolarity of the input signal.

The commutation circuitry may be configured to selectively couple one ofthe primary driver circuitry and the secondary driver circuitry to afirst terminal of the piezoelectric transducer, and to couple the otherof the primary driver circuitry and the secondary driver circuitry to asecond terminal of the piezoelectric transducer.

The commutation circuitry may be configured to selectively couple the ofthe primary driver circuitry and the secondary driver circuitry to thefirst terminal of the piezoelectric transducer, and to couple the otherof the primary driver circuitry and the secondary driver circuitry tothe second terminal of the piezoelectric transducer based on a polarityof the input signal.

The circuitry may further comprise first control circuitry forregulating operation of the primary driver and second control circuitryfor regulating operation of the secondary driver circuitry. A bandwidthof the second control circuitry may be greater than a bandwidth of thefirst control circuitry.

The secondary driver circuitry may be selectively operable based on aparameter of the input signal.

The parameter of the input signal may comprise one or more of: a signallevel; an envelope; and a frequency of the input signal.

The primary driver circuitry may be selectively operable based on asignal level or envelope of the input signal.

The primary driver circuitry and/or the secondary driver circuitry maybe selectively operable based on a mode of operation of the circuitry.

The secondary driver circuitry may be enabled in a first mode in whichthe input signal comprises an audio signal, and the secondary drivercircuitry may be disabled in a second mode in which the input signalcomprises a haptic signal or waveform.

In a further example, the primary driver circuitry and the secondarydriver circuitry both comprise charge pump circuitry.

In a further example, the primary driver circuitry comprises charge pumpcircuitry and the secondary driver circuitry comprises variable voltagepower supply circuitry.

According to a second aspect the invention provides integrated circuitrycomprising the circuitry of the first aspect.

According to a second aspect the invention provides a device comprisingthe circuitry of the first aspect.

The device may comprise, for example, a mobile telephone, a tablet orlaptop computer, a smart speaker, an accessory device, headphones,earphones or earbuds.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, strictly by way ofexample only, with reference to the accompanying drawings, of which:

FIG. 1 is a schematic diagram illustrating the concept of driving apiezoelectric transducer with charge;

FIG. 2 is a schematic diagram illustrating the concept of a helperamplifier arrangement;

FIG. 3 is a schematic diagram illustrating an example of drivercircuitry for driving a piezoelectric transducer;

FIG. 4 is a schematic diagram illustrating an alternative example ofdriver circuitry for driving a piezoelectric transducer;

FIG. 5 is a schematic diagram illustrating a further alternative exampleof driver circuitry for driving a piezoelectric transducer;

FIG. 6 is a schematic diagram illustrating a further alternative exampleof driver circuitry for driving a piezoelectric transducer;

FIG. 7 is a schematic diagram illustrating a control loop for the drivercircuitry of FIG. 4;

FIG. 8 is a schematic diagram illustrating an example of primary drivercircuitry for the driver circuitry of FIGS. 3-7;

FIG. 9 is a schematic diagram illustrating an example of secondarydriver circuitry for the driver circuitry of FIGS. 3-7;

FIG. 10 is a schematic diagram illustrating an alternative example ofsecondary driver circuitry for the driver circuitry of FIGS. 3-7;

FIG. 11 a schematic diagram illustrating commutator circuitry that canbe used to provide bipolar drive of a piezoelectric transducer; and

FIGS. 12-16 are schematic diagrams illustrating a further alternativeexamples of driver circuitry for driving a piezoelectric transducer.

DETAILED DESCRIPTION

Increasing the power efficiency of circuitry for driving a transducer,without adversely affecting output signal fidelity, is an ever-presentdesign goal, and many different techniques can be employed to increasepower efficiency. For example, one concept that may be used in audioapplications which use conventional audio output transducers (e.g.diaphragm-based loudspeakers) is the “helper amplifier”.

FIG. 2 is a schematic representation of an example helper amplifierarrangement 200 for driving a conventional audio output transducer 210,which in the illustrated arrangement is a loudspeaker.

The example helper amplifier arrangement 200 includes a primaryamplifier 220 which receives a positive power supply voltage VSup from apositive power supply rail 220. The primary amplifier 230 may be, forexample a switching amplifier such as a class D amplifier, which isconfigured to receive an audio input signal and to output a digitalprimary transducer driver signal based on the audio input signal to thetransducer 210, via a reconstruction filter 240.

The reconstruction filter 240 is configured to convert the digitalprimary transducer driver signal output by the primary amplifier 230into an analogue primary transducer driver signal suitable for drivingthe transducer 210. The reconstruction filter 240 comprises aninductance 242 coupled in series between an output of the primaryamplifier 230 and a first terminal of the transducer 210 and acapacitance 244 coupled in parallel with the transducer 210 between theinductance 242 and a reference voltage supply rail (e.g. a ground rail).A second terminal of the transducer 210 is coupled to a referencevoltage supply rail (e.g. a ground rail).

The helper amplifier arrangement 200 further includes a secondaryamplifier 250, which may be a linear amplifier such as, for example, aclass AB amplifier. The secondary amplifier 250 is configured to receivean error signal output by an error block 260, and to output a secondarytransducer driver signal to the transducer 210 based on the errorsignal.

The error block 260 is configured to receive, at a first input thereof,the input audio signal (or a signal indicative or representative of theinput audio signal) and to receive, at a second input thereof, theanalogue primary transducer driver signal (or a signal indicative orrepresentative of the analogue primary transducer driver signal), and togenerate the error signal based on a difference between the input audiosignal input and the analogue primary transducer driver signal.

In use of the helper amplifier arrangement 200 the primary amplifier 230provides the primary transducer driver signal to the transducer 210 in apower-efficient manner. The primary transducer driver signal provides amajority of the power required to drive the transducer 210 according tothe input audio signal. The secondary amplifier 250 provides thesecondary transducer driver signal to the transducer 210 to compensatefor any error in the analogue primary transducer driver signal. Althoughthe secondary amplifier 250 may be less power-efficient than the primaryamplifier 230, its overall effect on the power efficiency of helperamplifier arrangement 200 is small, because the secondary transducerdriver signal is small in comparison with the analogue primarytransducer driver signal, and so the power drawn from the positive powersupply rail 220 by the secondary amplifier 250 is small.

Thus the helper amplifier arrangement 200 can provide a high fidelityoutput signal for driving the transducer 210 with high power efficiency.However, the helper amplifier arrangement 200 of FIG. 2 is not suited todriving a piezoelectric transducer for a number of reasons, includingthe problems associated with hysteresis and creep discussed above.

FIG. 3 is a schematic representation of one example of driver circuitry300 for driving a piezoelectric transducer 310.

The driver circuitry 300 includes primary driver circuitry 320 fordriving the piezoelectric transducer 310. The primary driver circuitry320 comprises switching converter circuitry and may be, for example,variable voltage power supply circuitry, charge pump circuitry or someother form of switching converter circuitry.

The primary driver circuitry 320 receives a first positive supplyvoltage VSup from a first positive power supply rail 330 which may becoupled to a source of power such as a battery of a host deviceincorporating the driver circuitry 300 and the piezoelectric transducer310, either directly or via a voltage regulator or the like.

The primary driver circuitry 320 is configured to receive an inputsignal and to output a primary transducer drive signal, based on theinput signal, to cause charge to be transferred to and from thepiezoelectric transducer 310 to drive the piezoelectric transducer 310.A reservoir capacitance 322 (or a plurality of reservoir capacitances)is provided for storing charge that has been transferred from thepiezoelectric transducer 310 to facilitate “recycling” of charge,thereby reducing the need for additional charge to be provided from thepositive power supply rail 330.

The driver circuitry 300 further includes secondary driver circuitry 340for driving the piezoelectric transducer 310. The secondary drivercircuitry 340 also comprises switching converter circuitry and may be,for example, variable voltage power supply circuitry, charge pumpcircuitry or some other form of switching converter circuitry.

Thus, the present disclosure encompasses embodiments of driver circuitry300 in which the primary driver circuitry and the secondary drivercircuitry 320, 340 both comprise variable voltage power supplycircuitry, or in which the primary driver circuitry and the secondarydriver circuitry 320, 340 both comprise charge pump circuitry, as wellas embodiments in which one of the primary driver circuitry and thesecondary driver circuitry 320, 340 comprises variable voltage powersupply circuitry and the other of the secondary driver circuitry 340 andthe primary driver circuitry 320 comprises charge pump circuitry.

The secondary driver circuitry 340 receives a second positive supplyvoltage VBoost from a second positive power supply rail 360. The secondpositive power supply rail 360 may receive the second positive powersupply voltage VBoost from a boost converter or the like which isconfigured to generate the second positive supply voltage VBoost from alower voltage supply such as a supply voltage provided by the battery ofthe host device.

The driver circuitry 300 further includes an error block 350 configuredto receive, at a first input 352 thereof, the input signal (or a signalindicative or representative of the input signal) and to receive, at asecond input 354 thereof, a signal indicative or representative of theprimary transducer drive signal, e.g. a digital signal output by an ADC360 representing a voltage across the piezoelectric transducer 310, andto generate an error signal based on the input signal and the primarytransducer drive signal.

The secondary driver circuitry 340 is configured to receive the errorsignal output by the error block 350, and to output a secondarytransducer drive signal, based on the error signal, to cause charge tobe transferred to and from the piezoelectric transducer 310 in order tocompensate for any error between the input signal and the primarytransducer drive signal.

In the example illustrated in FIG. 3, an output of the primary drivercircuitry 320 is coupled to a first terminal 312 of the piezoelectrictransducer 310, and a second terminal 314 of the piezoelectrictransducer 310 is coupled to a reference voltage supply rail (e.g. aground rail). An output of the secondary driver circuitry 330 is alsocoupled to the first terminal 312 of the piezoelectric transducer 310.Thus the piezoelectric transducer 310 is coupled in parallel with theprimary and secondary driver circuitry 320, 340.

In this arrangement, particularly where the piezoelectric transducer 310is made up of a single layer or a small number of layers ofpiezoelectric material, the secondary driver circuitry 340 may berequired to provide a relatively large output voltage (e.g. of the orderof 100V) at its output in order to provide the charge required tocompensate for error in the primary transducer driver signal so as toproduce the required displacement of the piezoelectric transducer 310.In order to provide the required output voltage the supply voltageVBoost may need to be significantly higher than the output voltageprovided by the battery of the host device incorporating the circuitry300, and thus it may be necessary for the secondary supply voltageVBoost to be provided by a boost converter as described above.

FIG. 4 is a schematic representation of an alternative example of drivercircuitry 400 for driving a piezoelectric transducer 310. The drivercircuitry 400 is similar to the driver circuitry 300 of FIG. 3, and thuslike elements are denoted by like reference numerals in FIGS. 3 and 4.

The driver circuitry 400 of FIG. 4 differs from the driver circuitry 300of FIG. 3 in that the output of the primary driver circuitry 320 iscoupled to the first terminal 312 of the piezoelectric transducer 320and the output of the secondary driver circuitry 340 is coupled to thesecond terminal 314 of the piezoelectric transducer 310. Thus thepiezoelectric transducer 310 is coupled in series with the primarydriver circuitry and the secondary driver circuitry 340.

An advantage of the series coupling arrangement of the driver circuitry400 of FIG. 4, as compared to the parallel coupling arrangement of thedriver circuitry 300 of FIG. 3, is that, because the voltage across thepiezoelectric transducer 310, in use of the driver circuitry 400, isbased on the difference between the primary transducer drive signalvoltage and the secondary transducer drive signal voltage, a relativelysmall voltage output by the secondary driver circuitry 340 is sufficientto provide the necessary compensation for error in the primarytransducer drive signal. Accordingly the second positive power supplyvoltage VSup to the secondary driver circuitry 340 need not be high andcan thus be provided (directly or indirectly, e.g. via a voltageregulator) by the battery of the host device. As the supply voltage tothe secondary driver circuitry 340 is relatively low, it can use lowpower devices (e.g. transistors) which are physically small, which helpsto minimise the silicon area occupied by the secondary driver circuitry340 in an integrated circuit implementation of the driver circuitry 400.

FIG. 5 is a schematic representation of a further alternative example ofdriver circuitry 500 for driving a piezoelectric transducer 310. Thedriver circuitry 500 is similar in some respects to the driver circuitry400 of FIG. 4, and thus like elements are denoted by like referencenumerals in FIGS. 4 and 5. For the sake of clarity and brevity someelements of the driver circuitry 400 of FIG. 4 are not shown in FIG. 5.

The driver circuitry 500 of FIG. 5 differs from the driver circuitry 400of FIG. 4 in that it includes a “helper” capacitor 510, coupled betweena node 512, which is intermediate the second terminal 314 of thepiezoelectric transducer 310 and an output of the secondary drivercircuitry 340, and ground. Thus the helper capacitor 510 is coupled inseries between the piezoelectric transducer 310 and ground. Thecapacitance of the helper capacitor 510 may be of the order of ten timesthe nominal capacitance of the piezoelectric transducer 310.

The driver circuitry 500 includes an input signal path 520 which couplesan input node 522 of the driver circuitry 500 to an input of the primarydriver circuitry 320. The input signal path 520 includes adder circuitry524. An output signal path 530 couples an output of the primary drivercircuitry 320 to the first terminal 312 of the piezoelectric transducer310 at a node 532.

A primary driver feedback path 540 couples the output signal path 530 toa first input of first subtractor circuitry 542. A second input of thefirst subtractor circuitry 542 is coupled to an output of the addercircuitry 524. The first subtractor circuitry 542 is configured tooutput, to the primary driver circuitry 320, a primary driver feedbacksignal FBPRIM indicative of the difference between the voltage at theinput of the primary driver circuitry 320 and the voltage at the outputof the primary driver circuitry 320.

The driver circuitry 500 further includes an error signal path 550,which couples the node 532 to an input of the secondary driver circuitry320, via second subtractor circuitry 552. A first input of the secondsubtractor circuitry 552 is coupled to the input node 522 of the inputsignal path 520, and a second input of the second subtractor circuitry552 is coupled to the node 532. An output of the second subtractorcircuitry 552 is coupled to an input of the secondary driver circuitry340 so as to provide a first error signal Error1 to the secondary drivercircuitry 340.

The driver circuitry 500 further includes a secondary driver feedbackpath 560, which couples an output of the secondary driver circuitry 340to the secondary driver circuitry 340, via third subtractor circuitry562. Thus, the output of the secondary driver circuitry 340 is coupledto a first input of the third subtractor circuitry 562. A second inputof the third subtractor circuitry 562 receives a signal indicative of atarget voltage VH_TARGET across the helper capacitor 510. The thirdsubtractor circuitry 562 thus outputs, to the secondary driver circuitry340, a signal indicative of the difference between the target voltageVH_TARGET and the actual voltage VH across the helper capacitor 510.

The driver circuitry 500 further includes a filter signal path 570,which includes a low pass filter 572 having an input coupled to the node520 and an output coupled to a first input of the adder circuitry 524. Asecond input of the adder circuitry 524 receives an input voltage VSigrepresentative of a signal (e.g. an audio or haptic signal) to be outputby the piezoelectric transducer 310. The adder circuitry 524 isconfigured to add the signals received at its inputs and to output theresult of this addition. Thus, the adder circuitry 324 outputs a signalVSig+VH* (where VH* is a filtered version of the voltage VH across thehelper capacitor 510) to the primary driver circuitry 320.

The primary driver circuitry 320 is configured to output a primaryvoltage VPRIM_OUT, to the piezoelectric transducer 310 to transfercharge to the piezoelectric transducer 310 in order to causedisplacement of the piezoelectric transducer 310.

The secondary driver circuitry 340 is configured to selectively chargeor discharge the helper capacitor 510 in order to adjust a voltage VHacross the helper capacitor 510, to facilitate the transfer of charge toor from the piezoelectric transducer 310 to correct any error in theprimary output voltage VPRIM_OUT.

The instantaneous voltage VP across the piezoelectric transducer 310 asa result of the primary voltage VPRIM_OUT is equal to the differencebetween VPRIM_OUT and VH, i.e. VP-VPRIM_OUT-VH. Thus, the helpercapacitor 510 provides a “buffer” voltage VH that can be adjustedupwards or downwards (by appropriate action of the secondary drivercircuitry 340) in order to adjust the voltage VP across thepiezoelectric transducer 310 to compensate for error in the primaryvoltage VPRIM_OUT output by the primary driver circuitry 320.

In order for the displacement of the piezoelectric transducer 310 tocorrectly represent the input signal, the voltage VP should be equal tothe voltage VSig. Thus, VPRIM_OUT should be as close as possible toVSig+VH in order for the voltage VP to be equal to VSig (sinceVP=VPRIM_OUT-VH). As will be appreciated, however, if VH were simplyadded to VSig, then high-frequency transient changes in VH could lead toundesirable oscillation in the voltage VP. The low-pass filter circuitry572 is operative to attenuate such high-frequency transients, and toprovide the filtered version VH* of the voltage VH, which is added toVSig by the adder circuitry 524.

Thus, in the absence of any error, VPRIM_OUT=VSig+VH*, andVP=VPRIM_OUT-VH≈VSig.

The primary driver circuitry 320 is operative to adjust its outputsignal VPRIM_OUT based on the primary driver feedback signal FBPRIM tomaintain VPRIM_OUT as close to VSig+VH* as possible, thereby minimisingerror in VPRIM_OUT.

In order to correct any error in the voltage VP (i.e. to correct anydifference between VSig and VP), the secondary driver circuitry 340receives the first error signal Error1, which is indicative of thedifference between VP and VSig, from the subtractor circuitry 552, andadjusts the voltage VH across the helper capacitor 510 accordingly, byadding charge to or removing charge from the helper capacitor 510. Thusif VP is greater than VSig, charge can be added to the helper capacitor510 to increase the voltage VH to compensate. Similar, if VP is lessthan VSig charge can be removed from the helper capacitor 510 to reducethe voltage VH to compensate.

As will be appreciated, repeatedly correcting error in VP by reducingthe voltage VH across the helper capacitor 510 could eventually lead tothe voltage VH being reduced to zero, which would make it impossible toreduce VP further without a negative voltage supply.

To avoid this possibility the secondary driver feedback path 560provides a second error signal Error2, indicative of the differencebetween the target voltage VH_TARGET across the helper capacitor 510 andthe actual voltage VH across the helper capacitor 510, and the secondarydriver circuitry 340 is operative to adjust its output in order tominimise the second error signal Error2 in the long term. Thus thesecondary driver circuitry 340 can operate (almost) instantaneously toadjust the voltage VH in order to compensate for error in VPRIM_OUT toachieve the correct VP for a given input signal, but over a longerduration the secondary driver circuitry 340 is operative to maintain thevoltage VH across the helper capacitor 510 at the target voltageVH_TARGET, such that after any adjustment in VH to compensate for errorin VPRIM_OUT, VH returns to VH_TARGET.

As will be appreciated by those skilled in the art, the control loopimplemented by the error signal path 550 should operate more quickly(i.e. have a higher bandwidth) than the control loop implemented by thesecondary driver feedback path 560.

Additionally, the control loop implemented by the error signal path 550should operate more quickly (i.e. have a higher bandwidth) than thecontrol loop implemented by the primary driver feedback path 540, inorder to compensate for error in VPRIM_OUT quickly.

More generally, in all of the disclosed examples, a control loop thatcontrols or regulates the operation of the secondary driver circuitry340 will have a higher bandwidth than a control loop that controls orregulates the operation of the primary driver circuitry 320, in order tocompensate quickly for error in the primary driver signal output by theprimary driver circuitry 320.

FIG. 6 is a schematic representation of an alternative example of drivercircuitry 600 for driving a piezoelectric transducer 310. The drivercircuitry 600 is similar to the driver circuitry 400 of FIG. 3, and thuslike elements are denoted by like reference numerals in FIGS. 3 and 6.

In the driver circuitry 600 of FIG. 6, positive and negative powersupply voltages Vp, Vn supplied to the secondary driver circuitry 340 bythe primary driver circuitry 320 vary according to a level, envelope orother parameter of the input signal.

To this end, the primary driver circuitry 320, which in this examplecomprises or implements a variable voltage power supply, as described indetail below with reference to FIG. 8, may include detector circuitry610 that receives the input signal, detects a signal level (e.g. amagnitude), envelope or some other parameter of the input signal andcontrols the positive and negative power supply voltages Vp, Vn based onthe detected parameter of the input signal. The positive and negativepower supply voltages Vp, Vn are supplied to the secondary drivercircuitry via respective first (positive) and second (negative) supplyvoltage rails 620, 630.

The driver circuitry 600 further includes first and second supplycapacitors 640, 650. The first supply capacitor 640 (labelled Cp in FIG.6) is coupled between the first power supply rail 620 and ground, whilstthe second supply capacitor 650 (labelled Cn in FIG. 6) is coupledbetween the second power supply rail 630 and ground.

In the example illustrated in FIG. 6 the primary driver circuitry 320provides positive and negative voltages Vp, Vn to the secondary drivercircuitry 340. However, as will be appreciated by those of ordinaryskill in the art, if the secondary driver circuitry 340 were configuredto operate on, for example, a ground-referenced power supply, theprimary driver circuitry 320 would provide only a positive power supplyvoltage to the secondary driver circuitry 340. In such an arrangementonly a single supply capacitor would be required, coupled between thefirst (positive) power supply rail 620 and the ground.

The voltage(s) output by the primary driver circuitry 320 are dependentupon the detected signal level, envelope or other parameter of the inputsignal. Thus, if the detected signal level, envelope or other parameteris indicative that the input signal is increasing, the magnitude of thevoltage(s) output by the primary driver circuitry 320 to power thesecondary driver circuitry 340 increases. Conversely, if the detectedlevel, envelope or other parameter is indicative that the input signalis decreasing, the magnitude of the voltage(s) output by the primarydriver circuitry to power the secondary driver circuitry 340 decreases.

The secondary driver circuitry 340 is coupled to the primary drivercircuitry 320 so as to receive the variable positive and negativevoltages Vp, Vn as a power supply. The secondary driver circuitry 340 isalso operative to receive the error signal output by the error block 350and to provide a compensating second drive signal to drive thepiezoelectric transducer 310, as discussed above.

The use of the variable supply voltage(s) to power the secondary drivercircuitry 340 helps to improve the power efficiency of the secondarydriver circuitry 340 by reducing or minimising unnecessary headroom inthe supply voltage.

The reservoir capacitor 322 and the primary driver circuitry 320 can beused to improve further the power efficiency of the circuitry 600, bytransferring charge between the reservoir capacitor 322 and the firstand second supply capacitors 640, 650 as the supply requirements of thesecondary driver circuitry 340 change.

For example, when the level or envelope of the input signal isdecreasing there might be more charge stored in the first and secondsupply capacitors 640, 650 than is required to supply the secondarydriver circuitry 340 in order to provide the compensating second drivesignal to the piezoelectric transducer 310. Instead of wasting power bydischarging the first and second supply capacitors 640, 650 to ground inthis situation, the excess charge can be transferring to the reservoircapacitor 322, using a switch network and an inductor of the primarydriver circuitry 320, by first controlling the switch network toestablish a current path between the first or second supply capacitor640, 650 and ground via the inductor so as to cause a magnetic field todevelop around the inductor, and then controlling the switch network todecouple the first or second supply capacitor 640, 650 from the inductorand to establish a current path from the inductor to the reservoircapacitor 322, such that as the magnetic field around the inductorcollapses a current is induced which flows to the reservoir capacitor322, thus charging the reservoir capacitor 322.

Conversely, when the level or envelope of the input signal isincreasing, an increase in the amount of charge stored in the first andsecond supply capacitors 640, 650 may be required in order to supply thesecondary driver circuitry 340 to support the required output signallevel. The required increase can be achieved at least in part bytransferring stored charge from the reservoir capacitor 332, again usingthe switch network and the inductor of the primary driver circuitry 320,by first controlling the switch network to establish a current pathbetween the reservoir capacitor 322 and ground via the inductor so as tocause a magnetic field to develop around the inductor, and thencontrolling the switch network to decouple the reservoir capacitor 322from the inductor and to establish a current path from the inductor tothe first or second supply capacitor 640, 650, such that as the magneticfield around the inductor collapses a current is induced which flows tothe first or second supply capacitor 640, 650, thus charging the firstor second supply capacitor 640, 650.

Although in the example shown in FIG. 6 the primary and secondary drivercircuitry 320, 340 are coupled in parallel with the piezoelectrictransducer 310, those of ordinary skill in the art will appreciate thatthe principle of powering the secondary driver circuitry 340 using oneor more supply voltages which vary according to a signal level, envelopeor other parameter of the input signal is equally applicable to theseries-coupled arrangement shown in FIGS. 4 and 5, in which the primaryand secondary driver circuitry 320, 340 are coupled in series with thepiezoelectric transducer 310.

FIG. 7 is a schematic diagram illustrating driver circuitry 700including a control loop 710 for the secondary driver circuitry 340. Thecontrol loop 710 illustrated in FIG. 7 and described below is applicableto the driver circuitry 300, 400, 500, 600 of FIGS. 3-6.

The driver circuitry 700 is similar to the driver circuitry 400 of FIG.4, and so like elements are denoted by like reference numerals and willnot be described in detail here. The first and second supply rails 330,360 are not shown in FIG. 7 for reasons of clarity and brevity, butthose of ordinary skill in the art will readily appreciate that theprimary driver circuitry 320 and the secondary driver circuitry 340receive respective first and second supply voltages from respectivefirst and second positive supply rails.

The control loop 710 includes an analogue to digital converter (ADC) 720having an input terminal coupled to the second terminal 314 and anoutput coupled to the second input 354 of the error block 350. The ADC720 is configured to convert an analogue signal (e.g. a voltage acrossthe piezoelectric transducer 310) indicative of the charge on thepiezoelectric transducer 310 resulting from the primary drive signaloutput by the primary driver circuitry 320 into a digital signal that isoutput to the error block 350. The error block 350 receives, at itsfirst input 352, the input signal via a feedforward signal path, andoutputs a digital error signal indicative of the difference between theinput signal and signal output by the ADC 720 (and thus indicative of anerror between the input signal and the primary drive signal output bythe primary driver circuitry 320) to a loop filter 730.

The loop filter 730 (which may be, for example, a digital integrator orthe like) provides a filtered version of the error signal output by theerror block 350 to a delta-sigma digital to analogue converter 740,which in turn outputs a control signal to the secondary driver circuitry340 to control the operation of the secondary driver circuitry 340 toincrease or decrease the charge on the piezoelectric transducer 310 asnecessary to compensate for the error between the input signal and theprimary drive signal output by the primary driver circuitry 320.

The control loop 710 may also include a filter 750 in the feedforwardsignal path to the first input 352 of the error block 350, in order tointroduce a delay and transfer function to the input signal thatcorrespond to a delay and transfer function of the ADC 620 in thefeedback path between the second terminal 314 of the piezoelectrictransducer 310 and the second input 354 of the error block 350.

As indicated above in relation to the circuitry 500 of FIG. 5, in orderto compensate quickly for error in the primary drive signal output bythe primary driver circuitry the control loop 710 should have a greaterbandwidth than that of a control loop (not shown) that regulates theoperation of the primary driver circuitry 320.

In the examples described above the primary driver circuitry 320 and thesecondary driver circuitry 340 are described as operating continuouslyand simultaneously. In some examples, however, the secondary drivercircuitry 340 and/or the primary driver circuitry 320 may be selectivelyoperable (i.e. may be selectively enabled/activated ordisabled/deactivated) based on some predetermined condition.

For example, the secondary driver circuitry may be selectively operablebased on a parameter of an input signal such as a frequency or a levelsuch as an amplitude or envelope of the input signal.

At low input signal levels the output of the primary driver circuitry320 may be sufficiently accurate to drive the piezoelectric transducer310, and thus the secondary driver circuitry 340 may be disabled ordeactivated when the input signal level is below a first threshold, toreduce the power consumption of the driver circuitry. At higher inputsignal levels the primary driver circuitry 320 may not be able to outputa sufficiently accurate output signal to drive the piezoelectrictransducer 310 accurately, and so if the input signal level meets orexceeds the first threshold, the secondary driver circuitry 340 may beenabled or activated to compensate for error in the primary drive signaloutput by the primary driver circuitry 320.

Similarly, for input signals within a particular frequency range, theoutput of the primary driver circuitry 320 may be sufficiently accurateto drive the piezoelectric transducer 310, and thus the secondary drivercircuitry 340 may be disabled or deactivated when the input signalfrequency is within the particular frequency range, to reduce the powerconsumption of the driver circuitry. For signals outside the particularfrequency range the primary driver circuitry 320 may not be able tooutput a sufficiently accurate output signal to drive the piezoelectrictransducer 310 accurately, and so if the frequency of the input signalmoves outside the particular frequency range, the secondary drivercircuitry 340 may be enabled or activated to compensate for error in theprimary drive signal output by the primary driver circuitry 320.

Additionally or alternatively, in some examples the secondary drivercircuitry 340 and/or the primary driver circuitry 320 may be selectivelyoperable in different modes of operation of the driver circuitry. Forexample when the driver circuitry is operating in a first mode to outputan audio signal to the piezoelectric transducer, both the primary drivercircuitry 320 and the secondary driver circuitry 340 may both be enabledor activated, whereas when the driver circuitry is operating in a secondmode to output a haptic signal or waveform to the piezoelectrictransducer, the primary driver circuitry 320 may be enabled or activatedand the secondary driver circuitry 340 may be disabled or deactivated,to reduce power consumption.

In still further examples, the output signal power provided by theprimary driver circuitry 320 may not be required when the level (e.g.amplitude or envelope) of the input signal is very low. Thus, theprimary driver circuitry 320 may be disabled or deactivated when thelevel or envelope of the input signal is below a second threshold (whichmay be lower than the first threshold referred to above), such that thesecondary driver circuitry 340 drives the piezoelectric transducer 310.The primary driver circuitry 320 may be enabled or activated when theinput signal level meets or exceeds the second threshold.

FIG. 8 is a schematic representation of example variable voltage powersupply circuitry 800 for use as the primary or secondary drivercircuitry in the driver circuitry 300, 400, 500, 600.

The variable voltage power supply circuitry 800 includes the reservoircapacitance 322 for storing charge, an inductor 810 and a switch network820 (in this example comprising first to fifth controllable switches822-830, which may be, for example, MOSFET devices) for transferringcharge between the reservoir capacitance 322 and the piezoelectrictransducer 310. The reservoir capacitance 332 is shown in FIG. 8 as asingle capacitor, but it will be appreciated that the reservoircapacitance 332 may alternatively be provided by a plurality ofcapacitors coupled together.

The variable voltage power supply circuitry 800 is configured to receivepower from power supply circuitry 840 and to selectively provide chargeto the reservoir capacitance 322. The power supply circuitry 840 maycomprise boosted power supply circuitry configured to receive arelatively low voltage power supply, e.g. from a battery of the hostdevice, and to output a higher (boosted) power supply voltage VSup.

Although the variable voltage power supply circuitry 800 is shown asincluding only a single inductor 810 (and this may be preferable, tominimise the number of external components and thus reduce the cost andspace requirements of the primary driver circuitry 800), in someexamples there may be more than one inductor. For example, a firstinductor may be provided for transferring charge from the power supplycircuitry 840 to the reservoir capacitance 322 and a second inductor maybe provided for transferring charge from the reservoir capacitance 322to the piezoelectric transducer 310.

Further, where the secondary driver circuitry 340 is implemented usingthe variable voltage power supply circuitry 800, the inductor 810 may bephysically small (since the secondary driver circuitry is required toprovide only a relatively small voltage to compensate for error in theoutput of the primary driver circuitry) and thus, where the secondarypower supply circuitry is implemented as integrated circuitry, theinductor 810 may be embedded in the integrated circuitry, rather thanbeing provided as a separate off-chip component.

By contrast, where the primary driver circuitry is implemented using thevariable voltage power supply circuitry 800, the inductor 810 istypically physically large, i.e. too large to be embedded in integratedcircuitry, (since the primary driver circuitry is required to provide arelatively large output voltage) and thus, where the primary powersupply circuitry is implemented as integrated circuitry, the inductor810 typically cannot be embedded in the integrated circuitry, insteadbeing provided as a separate off-chip component.

The first switching device 822 is coupled between an output of the powersupply circuitry 840 and a first terminal the inductor 810.

The second switching device 824 is coupled between the first terminal ofthe inductor 810 and a reference voltage supply rail (e.g. a groundrail).

The third switching device 826 is coupled between a second terminal ofthe inductor 810 and the reference voltage supply rail.

The fourth switching device 828 is coupled between the second terminalof the inductor 810 and a first terminal of the reservoir capacitance332. A second terminal of the reservoir capacitance 332 is coupled tothe reference voltage supply rail.

The fifth switching device 830 is coupled between the first terminal ofthe inductor 810 and a first terminal 312 of the piezoelectrictransducer 310. A second terminal 314 of the piezoelectric transducer310 is coupled to the reference voltage supply rail.

The variable voltage power supply circuitry 800 further includes controlcircuitry 850, operable to control the switching devices 822-830 tocontrol the transfer of charge between the power supply circuitry 840,the reservoir capacitance 322 and the piezoelectric transducer 310.

On start-up of the variable voltage power supply circuitry 800 (or ahost device incorporating the primary driver circuitry 800), charge istransferred from the power supply circuitry 840 to the reservoircapacitance 322 to raise a voltage across the reservoir capacitance 322to a level that is suitable for driving the piezoelectric transducer310.

In a first phase of a charging process the first and third switches 822,826 are closed, in response to appropriate control signals transmittedby the control circuitry 850. This creates a current path though theinductor 810. As current flows through the inductor 810 a magnetic fielddevelops around it, storing energy.

In a second phase of the charging process, the first and third switches822, 826 are opened and the second and fourth switches 824, 828 areclosed, again in response to appropriate control signals transmitted bythe control circuitry 850. The magnetic field around the inductor 810collapses, inducing a current which flows from the inductor 810 to thereservoir capacitance 322, thereby charging the reservoir capacitance322.

The first and second phases are repeated until the voltage across thereservoir capacitance 322 has increased to a level that is suitable fordriving the piezoelectric transducer 310, as determined by the controlcircuitry 850 based on a feedback signal received from the piezoelectrictransducer 310. Once the reservoir capacitance 322 has been charged upto the desired level the first switch 822 is opened, thus decoupling thepower supply circuitry 840, such that the piezoelectric transducer 310can be driven by transferring charge from the reservoir capacitance 322.

When variable voltage power supply circuitry 800 is required to increasethe level of charge on the piezoelectric transducer 310, e.g. to drivethe piezoelectric transducer 310 to produce a transducer output, thevariable voltage power supply circuitry 800 again operates in twophases.

In a first phase of the charge transfer process the second and fourthswitches 824, 828 are closed, in response to appropriate control signalstransmitted by the control circuitry 850. A current path is thereforeestablished from the reservoir capacitance 322 through the inductor 810.As current flows through the inductor 810 a magnetic field developsaround it, storing energy.

In a second phase of the charge transfer process, the fifth switch 830is closed, and the second and fourth switches 824, 828 are opened, inresponse to appropriate control signals transmitted by the controlcircuitry 850. The magnetic field around the inductor 810 collapses,inducing a current which flows from the inductor 810 to thepiezoelectric transducer 310, thereby increasing the charge on thepiezoelectric transducer 310.

When variable voltage power supply circuitry 800 is required to reducethe level of charge on the piezoelectric transducer 310, charge can betransferred from the piezoelectric transducer 310 to the reservoircapacitance 322, such that the charge remains available for future use,rather than being lost. This improves the efficiency of the primarydriver circuitry 800.

The process of transferring charge from the piezoelectric transducer 310to the reservoir capacitance 322 occurs in two phases.

In a first phase the third and fifth switches 826, 830 are closed, inresponse to appropriate control signals transmitted by the controlcircuitry 850. A current path is therefore established from thepiezoelectric transducer 310 through the inductor 810. As current flowsthrough the inductor 810 a magnetic field develops around it, storingenergy.

In a second phase the second and fourth switches 824, 828 are closed,and the third and fifth switches 826, 830 are opened, in response toappropriate control signals transmitted by the control circuitry 850.The magnetic field around the inductor 810 collapses, inducing a currentwhich flows to the reservoir capacitance 322, thus charging thereservoir capacitance 322.

Thus in variable voltage power supply circuitry 800 the piezoelectrictransducer 310 can be driven by transferring charge to it from thereservoir capacitance 322, and charge can be recycled between thepiezoelectric transducer 310 and the reservoir capacitance 322 toimprove power efficiency. The power supply circuitry 840 provides theinitial charge to the reservoir capacitance 322 during the chargingprocess and occasionally or periodically tops up or recharges thereservoir capacitance 322 as necessary.

FIG. 9 is a schematic representation of one example of charge pumpcircuitry 900 for use in the driver circuitry 300, 400, 500, 600.

The charge pump circuitry 900 includes a flying capacitance 910, whichmay be a fixed capacitor or a variable capacitance (e.g. a bank ofcapacitances coupled in parallel, each capacitance being selectable bymeans of one or more switching devices so as to implement a capacitanceof a desired value) and a switch network 920 comprising first to fourthcontrollable switch devices 922-928, which may be, for example, MOSFETdevices. An output terminal 930 of the secondary driver circuitry 900 iscoupled to the first terminal 312 of the piezoelectric transducer 310.

The first controllable switch device 922 is coupled between the positivesupply rail 360 and a first terminal of the flying capacitance 910. Thesecond controllable switch device 924 is coupled between a secondterminal of the flying capacitance 910 and a negative supply voltagerail 940. The third controllable switch device 926 is coupled betweenthe first terminal of the flying capacitance 910 and the output terminal930 of the charge pump circuitry 900, and the fourth controllable switchdevice 928 is coupled between the second terminal of the flyingcapacitance 910 and the output terminal 930.

The charge pump circuitry 900 further includes control circuitry 950which is operative to control the controllable switching devices 922-928to charge the flying capacitance 910 and to transfer charge to and fromthe piezoelectric transducer 310 in a manner that will be apparent tothose of ordinary skill in the art.

Where the charge pump circuitry 900 is used as the secondary drivercircuitry, the flying capacitance 910 can be relatively small, since thesecondary driver circuitry is required to provide only a small outputvoltage to compensate for error in the signal output by the primarydriver circuitry. However, where the charge pump circuitry 900 used asthe primary driver circuitry, the flying capacitance 910 is typicallymuch larger, since the primary driver circuitry is required to provide arelatively large output voltages.

FIG. 10 is a schematic representation of an alternative example ofcharge pump circuitry 1000 for use in the driver circuitry 300, 400,500, 600.

The charge pump circuitry 1000 includes a flying capacitance 1010, whichagain may be a fixed capacitor or a variable capacitance (e.g. a bank ofcapacitances coupled in parallel, each capacitance being selectable bymeans of one or more switching devices so as to implement a capacitanceof a desired value) and a switch network 1020 comprising first andsecond controllable switch devices 1022, 1024, which may be, forexample, MOSFET devices. An output terminal 1030 of the charge pumpcircuitry 1000 is coupled to the first terminal 312 of the piezoelectrictransducer 310.

The first controllable switch device 1022 is coupled between thepositive supply rail 360 and a first terminal of the flying capacitance1010. The second controllable switch device 1024 is coupled between thefirst terminal of the flying capacitance 1010 and a negative supplyvoltage rail 1040.

The charge pump circuitry 1000 further includes control circuitry 1050which is operative to control the controllable switching devices 1022,1024 to charge the flying capacitance 1010 and to transfer charge to andfrom the piezoelectric transducer 310 in a manner that will be apparentto those of ordinary skill in the art.

Again, where the charge pump circuitry 1000 is used as the secondarydriver circuitry, the flying capacitance 1010 can be relatively small,since the secondary driver circuitry is required to provide only a smalloutput voltage to compensate for error in the signal output by theprimary driver circuitry. However, where the charge pump circuitry 1000used as the primary driver circuitry, the flying capacitance 1010 istypically much larger, since the primary driver circuitry is required toprovide a relatively large output voltages.

In the examples described above with reference to FIGS. 3 to 6 each ofthe first and second terminals 312, 314 of the piezoelectric transducer310 is permanently coupled to either the output of the primary drivercircuitry 320, or to the output of the secondary driver circuitry 340 orto the reference voltage (e.g. ground) rail.

It may be advantageous, in the series-connected transducer arrangementsillustrated in FIGS. 4, 5 and 7, to be able to select which of theterminals 312, 314 of the piezoelectric transducer 310 is coupled to theoutput of the primary driver circuitry 320 and which is coupled to theoutput of the secondary driver circuitry 340, e.g. depending upon thepolarity of an input signal to the driver circuitry. This can beachieved by using commutator circuitry 1100 coupled to the piezoelectrictransducer 310, as will now be described with reference to FIG. 11.

The commutator circuitry 1100 includes a switch network 1100, which inthe illustrated example includes first to fourth controllable switches1112-1118. The commutator circuitry 1100 is coupled to control circuitry1130 so as to receive control signals for controlling the operation ofthe controllable switches 1112-1118 according to the input signal.

In use of the commutator circuitry 1100 in implementations which couplethe piezo-electric transducer 310 in series with the primary andsecondary driver circuitry 320, 340, the first controllable switch 1112is coupled between a first node 1120 of the switch network 1110 and afirst terminal 312 of the piezoelectric transducer 310. The first node1120 of the switch network 1110 is coupled to an output of the primarydriver circuitry 320.

The second controllable switch 1114 is coupled between the firstterminal 312 of the piezoelectric transducer 310 and a second node 1140of the switch network 1110. The second node 1140 of the switch network1110 is coupled to an output of the secondary driver circuitry 340.

The third controllable switch 1116 is coupled between the first node1120 of the switch network 1110 and a second terminal 314 of thepiezoelectric transducer 310.

The fourth controllable switch 1118 is coupled between the secondterminal 314 of the piezoelectric transducer 310 and the second node1140 of the switch network 1110.

By selectively opening and closing the controllable switches 1112-1118one of the first and second terminals 312, 314 of the piezoelectrictransducer 310 can be coupled to the output of either the primary drivercircuitry 320 or the secondary driver circuitry 340, and the other ofthe first and second terminals 312, 314 of the piezoelectric transducer310 can be coupled to either the secondary driver circuitry 340 or tothe primary driver circuitry 340.

For example, closing the first and fourth switches 1112, 1118 andopening the second and third switches 1114, 1116 will cause the firstterminal 312 of the piezoelectric transducer 310 to be coupled to theoutput of the primary driver circuitry 320 and the second terminal 314of the piezoelectric transducer 310 to be coupled to the output of thesecondary driver circuitry 340. Alternatively, opening the first andfourth switches 1112, 1118 and closing the second and third switches1114, 1116 will cause the first terminal 312 of the piezoelectrictransducer 310 to be coupled to the output of the secondary drivercircuitry 340 and the second terminal 314 of the piezoelectrictransducer 310 to be coupled to the output of the primary drivercircuitry 320.

In the parallel-connected transducer arrangements illustrated in FIGS. 3and 6, it may be beneficial to be able to select which of the terminals312, 314 of the piezoelectric transducer 310 is coupled to the outputsof the primary and secondary driver circuitry 320, 340 and which iscoupled to ground (or some other reference voltage), e.g. depending uponthe polarity of an input signal to the driver circuitry. The commutatorcircuitry 1100 of FIG. 11 can also be used for this purpose, as will nowbe described.

In use of the commutator circuitry 1100 in implementations which couplethe piezo-electric transducer 310 in parallel with the primary andsecondary driver circuitry 320, 340, the first controllable switch 1112is coupled between a first node 1120 of the switch network 1110 and afirst terminal 312 of the piezoelectric transducer 310. The first node1120 of the switch network 1110 is coupled to an output of the primarydriver circuitry 320 and to an output of the secondary driver circuitry340.

The second controllable switch 1114 is coupled between the firstterminal 312 of the piezoelectric transducer 310 and a second node 1140of the switch network 1110. The second node 1140 of the switch network1110 is coupled to ground or some other reference voltage supply (e.g. ahelper capacitor of the kind described above with reference to FIG. 5).

The third controllable switch 1116 is coupled between the first node1120 of the switch network 1110 and a second terminal 314 of thepiezoelectric transducer 310.

The fourth controllable switch 1118 is coupled between the secondterminal 314 of the piezoelectric transducer 310 and the second node1140 of the switch network 1110.

By selectively opening and closing the controllable switches 1112-1118one of the first and second terminals 312, 314 of the piezoelectrictransducer 310 can be coupled to the outputs of the primary drivercircuitry 320 and the secondary driver circuitry 340, and the other ofthe first and second terminals 312, 314 of the piezoelectric transducer310 can be coupled to the reference voltage supply (e.g. ground).

For example, closing the first and fourth switches 1112, 1118 andopening the second and third switches 1114, 1116 will cause the firstterminal 312 of the piezoelectric transducer 310 to be coupled to theoutputs of the primary driver circuitry 320 and the secondary drivercircuitry 340, and the second terminal 314 of the piezoelectrictransducer 310 to be coupled to the reference voltage supply.Alternatively, opening the first and fourth switches 1112, 1118 andclosing the second and third switches 1114, 1116 will cause the firstterminal 312 of the piezoelectric transducer 310 to be coupled to thereference voltage supply and the second terminal 314 of thepiezoelectric transducer 310 to be coupled to the outputs of the primarydriver circuitry 320 and the secondary driver circuitry 340.

In some examples of the driver circuitry disclosed herein, a singlereservoir capacitor 322 may be shared between the primary and secondarydriver circuitry 320, 340. For example, where both the primary drivercircuitry 320 and the secondary driver circuitry 340 comprise variablevoltage power supply circuitry 800 of the kind described above withreference to FIG. 8, a single reservoir capacitor 322 may be provided,which is shared by the primary driver circuitry 320 and the secondarydriver circuitry 340 for the purpose of charge recirculation. Thisavoids the need for the primary driver circuitry 320 and the secondarydriver circuitry 340 each to have a reservoir capacitor, which reducesthe cost and area of the driver circuitry.

In other examples in which both the primary driver circuitry 320 and thesecondary driver circuitry 340 comprise variable voltage power supplycircuitry 800 of the kind described above with reference to FIG. 8, onlythe primary driver circuitry 320 may be provided with a reservoircapacitor 332. Again, this reduces the cost and area of the drivercircuitry, albeit at the cost of slightly reduced efficiency as chargecannot be recirculated in the secondary driver circuitry 340.

As a further alternative in which both the primary driver circuitry 320and the secondary driver circuitry 340 comprise variable voltage powersupply circuitry 800 of the kind described above with reference to FIG.8, the primary driver circuitry 320 and the secondary driver circuitry340 may each be provided with a reservoir capacitor 332. This maximisedpower efficiency, by permitting charge recirculation in both the primarydriver circuitry 320 and the secondary driver circuitry 340, at the costof increased component count, cost and circuit area.

FIG. 12 schematically illustrates an alternative example of drivercircuitry 1200 for driving a piezoelectric transducer 310. The drivercircuitry 1200 is similar in some respects to the driver circuitry 500of FIG. 5, and thus like elements are denoted by like reference numeralsin FIGS. 5 and 12. For the sake of clarity and brevity some elements ofthe driver circuitry 500 of FIG. 5 are not shown in FIG. 12.

The driver circuitry 1200 includes first and second “helper” capacitors1210, 1220. The first helper capacitor 1210 is coupled in parallel withthe piezoelectric transducer 310, between a node 1212 intermediate theoutput of the primary driver circuitry 320 and the first terminal 312 ofthe piezoelectric transducer 310 and ground. The second helper capacitor1220 is coupled in parallel with the piezoelectric transducer 310,between a node 1222 intermediate the output of the secondary drivercircuitry 340 and the first terminal 312 of the piezoelectric transducer310 and ground.

The first helper capacitor 1210 can be selectively charged anddischarged by the primary driver circuitry 320 in order to adjust avoltage VH1 across the first helper capacitor 1210, to facilitate thetransfer of charge to or from the piezoelectric transducer 310 based onthe primary driver signal output by the primary driver circuitry 320.

The second helper capacitor 1220 can be selective charged and dischargedby the secondary driver circuitry 340 in order to adjust a voltage VH2across the second helper capacitor 1220, to facilitate the transfer ofcharge to or from the piezoelectric transducer 310 to correct any errorin the primary driver signal output by the primary driver circuitry 320.

As will be appreciated by those of ordinary skill in the art, thecircuitry 1200 includes appropriate input, output, feedback and errorand filter signal paths, of the kind described above with reference toFIG. 5, for example, to provide the necessary control of the primary andsecondary driver circuitry 320, 340 in order to ensure that thedisplacement of the piezoelectric transducer 310 represents the inputsignal to the circuitry 1200 as accurately as possible.

FIG. 13 schematically illustrates a further alternative example ofdriver circuitry 1300 for driving a piezoelectric transducer 310. Thedriver circuitry 1300 is similar in some respects to the drivercircuitry 400 of FIG. 4, and thus like elements are denoted by likereference numerals in FIGS. 4 and 13. For the sake of clarity andbrevity some elements of the driver circuitry 400 of FIG. 4 are notshown in FIG. 13.

The driver circuitry 1300 includes a “helper” capacitor 1310. The helpercapacitor 1310 is coupled in series between the output of the secondarydriver circuitry 340 and the first terminal 312 of the piezoelectrictransducer 310.

The helper capacitor 1310 can be selectively charged and discharged bythe secondary driver circuitry 340 in order to adjust a voltage VHacross the helper capacitor 1310, to facilitate the transfer of chargeto or from the piezoelectric transducer 310 to correct any error in theprimary driver signal output by the primary driver circuitry 320.

Again, as will be appreciated by those of ordinary skill in the art, thecircuitry 1300 includes appropriate input, output, feedback and errorand filter signal paths, of the kind described above with reference toFIG. 5, for example, to provide the necessary control of the primary andsecondary driver circuitry 320, 340 in order to ensure that thedisplacement of the piezoelectric transducer 310 represents the inputsignal to the circuitry 1300 as accurately as possible.

FIG. 14 schematically illustrates a further alternative example ofdriver circuitry 1400 for driving a piezoelectric transducer 310. Thedriver circuitry 1400 is similar in some respects to the drivercircuitry 1300 of FIG. 13, and thus like elements are denoted by likereference numerals in FIGS. 13 and 14.

The driver circuitry 1400 includes a first “helper” capacitor 1410,which is coupled in series between the output of the secondary drivercircuitry 340 and the first terminal 312 of the piezoelectric transducer310 and ground.

The driver circuitry 1400 further includes a second “helper” capacitor1420, which is coupled in parallel with the piezoelectric transducer310, between the output of the secondary driver circuitry 340 andground.

The first and second helper capacitors 1410, 1420 can be selectivelycharged and discharged by the secondary driver circuitry 340 in order toadjust voltages VH1, VH2 across the first and second helper capacitors1410, 1420 respectively, to facilitate the transfer of charge to or fromthe piezoelectric transducer 310 to correct any error in the primarydriver signal output by the primary driver circuitry 320.

Again, as will be appreciated by those of ordinary skill in the art, thecircuitry 1400 includes appropriate input, output, feedback and errorand filter signal paths, of the kind described above with reference toFIG. 5, for example, to provide the necessary control of the primary andsecondary driver circuitry 320, 340 in order to ensure that thedisplacement of the piezoelectric transducer 310 represents the inputsignal to the circuitry 1400 as accurately as possible.

FIG. 15 schematically illustrates a further alternative example ofdriver circuitry 1500 for driving a piezoelectric transducer 310. Thedriver circuitry 1500 is similar in some respects to the drivercircuitry 1200 of FIG. 12, and thus like elements are denoted by likereference numerals in FIGS. 12 and 15.

The driver circuitry 1500 includes a “helper” capacitor 1510 coupled inparallel with the piezoelectric transducer 310, between a node 1512intermediate the output of the primary driver circuitry 320 and thefirst terminal 312 of the piezoelectric transducer 310 and ground.

The helper capacitor 1510 can be selectively charged and discharged bythe primary driver circuitry 320 in order to adjust a voltage VH acrossthe helper capacitor 1510, to facilitate the transfer of charge to orfrom the piezoelectric transducer 310 based on the primary driver signaloutput by the primary driver circuitry 320.

As will be appreciated by those of ordinary skill in the art, thecircuitry 1500 includes appropriate input, output, feedback and errorand filter signal paths, of the kind described above with reference toFIG. 5, for example, to provide the necessary control of the primary andsecondary driver circuitry 320, 340 in order to ensure that thedisplacement of the piezoelectric transducer 310 represents the inputsignal to the circuitry 1500 as accurately as possible.

FIG. 16 schematically illustrates a further alternative example ofdriver circuitry 1600 for driving a piezoelectric transducer 310. Thedriver circuitry 1600 is similar in some respects to the drivercircuitry 1200 of FIG. 12, and thus like elements are denoted by likereference numerals in FIGS. 12 and 16.

The driver circuitry 1600 includes a “helper” capacitor 1610 coupled inparallel with the piezoelectric transducer 310, between a node 1612intermediate the output of the secondary driver circuitry 340 and thefirst terminal 312 of the piezoelectric transducer 310 and ground.

The helper capacitor 1610 can be selectively charged and discharged bythe secondary driver circuitry 340 in order to adjust a voltage VHacross the helper capacitor 1610, to facilitate the transfer of chargeto or from the piezoelectric transducer 310 in order to correct anyerror in the primary driver signal output by the primary drivercircuitry 320.

As will be appreciated by those of ordinary skill in the art, thecircuitry 1600 includes appropriate input, output, feedback and errorand filter signal paths, of the kind described above with reference toFIG. 5, for example, to provide the necessary control of the primary andsecondary driver circuitry 320, 340 in order to ensure that thedisplacement of the piezoelectric transducer 310 represents the inputsignal to the circuitry 1600 as accurately as possible.

As will apparent from the foregoing discussion, the circuitry of thepresent disclosure provides driver circuitry for driving a piezoelectrictransducer which includes power-efficient primary driver circuitry forproviding a majority of the power required to drive the piezoelectrictransducer, and accurate secondary driver circuitry which provides asmall (relatively to the primary drive signal provided by the primarydriver circuitry) secondary drive signal to correct or compensate, atleast partially, for error in the primary drive signal, thus providingaccurate and power-efficient driving of the piezoelectric transducer.

Embodiments may be implemented as an integrated circuit which in someexamples could be a codec or audio DSP or similar. Embodiments may beincorporated in an electronic device, which may for example be aportable device and/or a device operable with battery power. The devicecould be a communication device such as a mobile telephone or smartphoneor similar. The device could be a computing device such as a notebook,laptop or tablet computing device. The device could be a wearable devicesuch as a smartwatch. The device could be a device with voice control oractivation functionality such as a smart speaker. In some instances thedevice could be an accessory device such as a headset, headphones,earphones, earbuds or the like to be used with some other product.

The skilled person will recognise that some aspects of theabove-described apparatus and methods, for example the discovery andconfiguration methods may be embodied as processor control code, forexample on a non-volatile carrier medium such as a disk, CD- or DVD-ROM,programmed memory such as read only memory (Firmware), or on a datacarrier such as an optical or electrical signal carrier. For manyapplications, embodiments will be implemented on a DSP (Digital SignalProcessor), ASIC (Application Specific Integrated Circuit) or FPGA(Field Programmable Gate Array). Thus the code may comprise conventionalprogram code or microcode or, for example code for setting up orcontrolling an ASIC or FPGA. The code may also comprise code fordynamically configuring re-configurable apparatus such asre-programmable logic gate arrays. Similarly the code may comprise codefor a hardware description language such as Verilog™ or VHDL (Very highspeed integrated circuit Hardware Description Language). As the skilledperson will appreciate, the code may be distributed between a pluralityof coupled components in communication with one another. Whereappropriate, the embodiments may also be implemented using code runningon a field-(re)programmable analogue array or similar device in order toconfigure analogue hardware.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference numerals or labels in the claims shall not be construed soas to limit their scope.

As used herein, when two or more elements are referred to as “coupled”to one another, such term indicates that such two or more elements arein electronic communication or mechanical communication, as applicable,whether connected indirectly or directly, with or without interveningelements.

This disclosure encompasses all changes, substitutions, variations,alterations, and modifications to the example embodiments herein that aperson having ordinary skill in the art would comprehend. Similarly,where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to the exampleembodiments herein that a person having ordinary skill in the art wouldcomprehend. Moreover, reference in the appended claims to an apparatusor system or a component of an apparatus or system being adapted to,arranged to, capable of, configured to, enabled to, operable to, oroperative to perform a particular function encompasses that apparatus,system, or component, whether or not it or that particular function isactivated, turned on, or unlocked, as long as that apparatus, system, orcomponent is so adapted, arranged, capable, configured, enabled,operable, or operative. Accordingly, modifications, additions, oromissions may be made to the systems, apparatuses, and methods describedherein without departing from the scope of the disclosure. For example,the components of the systems and apparatuses may be integrated orseparated. Moreover, the operations of the systems and apparatusesdisclosed herein may be performed by more, fewer, or other componentsand the methods described may include more, fewer, or other steps.Additionally, steps may be performed in any suitable order. As used inthis document, “each” refers to each member of a set or each member of asubset of a set.

Although exemplary embodiments are illustrated in the figures anddescribed below, the principles of the present disclosure may beimplemented using any number of techniques, whether currently known ornot. The present disclosure should in no way be limited to the exemplaryimplementations and techniques illustrated in the drawings and describedabove.

Unless otherwise specifically noted, articles depicted in the drawingsare not necessarily drawn to scale.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding the disclosureand the concepts contributed by the inventor to furthering the art, andare construed as being without limitation to such specifically recitedexamples and conditions. Although embodiments of the present disclosurehave been described in detail, it should be understood that variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the disclosure.

Although specific advantages have been enumerated above, variousembodiments may include some, none, or all of the enumerated advantages.Additionally, other technical advantages may become readily apparent toone of ordinary skill in the art after review of the foregoing figuresand description.

To aid the Patent Office and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims or claimelements to invoke 35 U.S.C. § 112(f) unless the words “means for” or“step for” are explicitly used in the particular claim.

1. Circuitry for driving a piezoelectric transducer based on an inputsignal, the circuitry comprising: primary driver circuitry configured toreceive the input signal and to output a primary driving signal to thepiezoelectric transducer based on the input signal; and secondary drivercircuitry configured to receive an error signal indicative of an errorbetween the input signal and the primary driving signal and to output asecondary driving signal to the piezoelectric transducer based on theerror signal, wherein the primary driver circuitry and the secondarydriver circuitry both comprise switching converter circuitry. 2.Circuitry according to claim 1, wherein the primary driver circuitrycomprises variable voltage power supply circuitry and the secondarydriver circuitry comprises charge pump circuitry.
 3. Circuitry accordingto claim 2, wherein the variable voltage power supply circuitrycomprises a switch network, an inductor and a reservoir capacitor. 4.Circuitry according to claim 2, wherein the charge pump circuitrycomprises a switch network and a flying capacitor.
 5. Circuitryaccording to claim 4, wherein the flying capacitor is variable. 6.Circuitry according to claim 4, wherein the charge pump circuitry isconfigured to receive a power supply that varies based on a parameter ofthe input signal.
 7. Circuitry according to claim 6, wherein thevariable voltage power supply circuitry is configured to provide thepower supply to the charge pump circuitry.
 8. Circuitry according toclaim 7, wherein the variable voltage power supply circuitry comprisesdetector circuitry configured to detect a level, envelope or otherparameter of the input signal and to control the power supply voltageprovided to the charge pump circuitry based on the detected level,envelope or other parameter.
 9. Circuitry according to claim 3, whereinthe charge pump circuitry comprises one or more supply capacitors, andwherein the switch network is operable to transfer charge between thereservoir capacitor and the one or more supply capacitors.
 10. Circuitryaccording to claim 1, wherein the primary driver circuitry comprisesfirst variable voltage power supply circuitry and the secondary drivercircuitry comprises second variable voltage power supply circuitry. 11.Circuitry according to claim 10, wherein the primary driver circuitryand the secondary driver circuitry each comprise a switch network and aninductor, wherein the inductor of the secondary driver circuitry issmaller than the inductor of the primary driver circuitry.
 12. Circuitryaccording to claim 11, wherein the inductor of the secondary drivercircuitry is embedded in integrated circuitry that implements thecircuitry.
 13. Circuitry according to claim 10, wherein the firstvariable voltage power supply circuitry comprises a first reservoircapacitor for storing charge.
 14. Circuitry according to claim 13,wherein the first reservoir capacitor is shared by the first variablevoltage power supply circuitry and the second variable voltage powersupply circuitry.
 15. Circuitry according to claim 13, wherein thesecond variable voltage power supply circuitry comprises a secondreservoir capacitor for storing charge.
 16. Circuitry according to claim1, further comprising a helper capacitor configured to receive chargefrom the primary driver circuitry or the secondary driver circuitry inorder to adjust a voltage across the helper capacitor.
 17. Circuitryaccording to claim 16, wherein the helper capacitor is coupled: inseries between the piezoelectric transducer and ground; or. in seriesbetween an output of the secondary driver circuitry and a first terminalof the piezoelectric transducer; or in parallel with the piezoelectrictransducer, between an output of the secondary driver circuitry andground; or in parallel with the piezoelectric transducer, between anoutput of the primary driver circuitry and ground.
 18. Circuitryaccording to claim 16, wherein the circuitry comprises: a first helpercapacitor coupled in parallel with the piezoelectric transducer, betweenan output of the secondary driver circuitry and ground; and a secondhelper capacitor coupled in parallel with the piezoelectric transducer,between an output of the primary driver circuitry and ground. 19.Circuitry according to claim 16, wherein the circuitry comprises: afirst helper capacitor coupled in parallel with the piezoelectrictransducer, between an output of the secondary driver circuitry andground; and a second helper capacitor coupled in in series between anoutput of the secondary driver circuitry and a first terminal of thepiezoelectric transducer.
 20. Circuitry according to claim 1, whereinthe primary driver circuitry is configured to be coupled to a terminalof the piezoelectric transducer and the secondary driver circuitry isconfigured to be coupled to the same terminal of the piezoelectrictransducer.
 21. Circuitry according to claim 1, wherein the primarydriver circuitry is configured to be coupled to a first terminal of thepiezoelectric transducer and the secondary driver circuitry isconfigured to be coupled to a second terminal of the piezoelectrictransducer.
 22. Circuitry according to claim 16, further comprisingcommutation circuitry configured to selectively couple one of a firstterminal and a second terminal of the piezoelectric transducer to theprimary driver circuitry and the secondary driver circuitry, and tocouple the other of the first terminal and the second terminal of thepiezoelectric transducer to a reference voltage supply.
 23. Circuitryaccording to claim 22, wherein the commutation circuitry is configuredto selectively couple one of a first terminal and a second terminal ofthe piezoelectric transducer to the primary driver circuitry and thesecondary driver circuitry, and to couple the other of the firstterminal and the second terminal of the piezoelectric transducer to areference voltage supply based on a polarity of the input signal. 24.Circuitry according to claim 22, wherein the commutation circuitry isconfigured to selectively couple one of the primary driver circuitry andthe secondary driver circuitry to a first terminal of the piezoelectrictransducer, and to couple the other of the primary driver circuitry andthe secondary driver circuitry to a second terminal of the piezoelectrictransducer.
 25. Circuitry according to claim 24, wherein the commutationcircuitry is configured to selectively couple the of the primary drivercircuitry and the secondary driver circuitry to the first terminal ofthe piezoelectric transducer, and to couple the other of the primarydriver circuitry and the secondary driver circuitry to the secondterminal of the piezoelectric transducer based on a polarity of theinput signal.
 26. Circuitry according to claim 1, further comprisingfirst control circuitry for regulating operation of the primary driverand second control circuitry for regulating operation of the secondarydriver circuitry, wherein a bandwidth of the second control circuitry isgreater than a bandwidth of the first control circuitry.
 27. Circuitryaccording to claim 1, wherein the secondary driver circuitry isselectively operable based on a parameter of the input signal. 28.Circuitry according to claim 27, wherein the parameter of the inputsignal comprises one or more of: a signal level; an envelope; and afrequency of the input signal.
 29. Circuitry according to claim 1,wherein the primary driver circuitry is selectively operable based on asignal level or envelope of the input signal.
 30. Circuitry according toclaim 1, wherein the primary driver circuitry and/or the secondarydriver circuitry is selectively operable based on a mode of operation ofthe circuitry.
 31. Circuitry according to claim 30, wherein thesecondary driver circuitry is enabled in a first mode in which the inputsignal comprises an audio signal, and wherein the secondary drivercircuitry is disabled in a second mode in which the input signalcomprises a haptic signal or waveform.
 32. Circuitry according to claim1 wherein the primary driver circuitry and the secondary drivercircuitry both comprise charge pump circuitry.
 33. Circuitry accordingto claim 1 wherein the primary driver circuitry comprises charge pumpcircuitry and the secondary driver circuitry comprises variable voltagepower supply circuitry.
 34. Integrated circuitry comprising thecircuitry of claim
 1. 35. A device comprising the circuitry of claim 1.36. A device according to claim 35, wherein the device comprises amobile telephone, a tablet or laptop computer, a smart speaker, anaccessory device, headphones, earphones or earbuds.